To cement a casing in a well bore, the volume of cement slurry that is needed is calculated taking into account the dimensional characteristics of the casing and the borehole, the depth of the uppermost float valve where a displacement plug is expected to seat and seal, and the overall volumetric efficiency of the displacement system including the pump, valve, piping and the like. If the displacement plug does not in fact reach the seat when the calculated total displacement volume has been pumped, the typical procedure is to stop at this point for fear of overdisplacing the cement, that is, displacing all of it outside the casing. Of course overdisplacement creates a problem situation that drilling supervisors seek to avoid.
However being overly cautions respecting overdisplacement sometimes results in underdisplacement where a large volume of cement is left inside the casing. When the cement hardens, a lengthy column remains that must be drilled out, which is a time consuming and expensive process, as is the repair to the well when of an overdisplacement has occurred.
The use of cementing plugs is known. Prior systems are perhaps best described in U.S. Pat. No. 4,427,065 issued to James S. Watson. This patent discloses a cylindrical cementing plug container assembly which is loaded with one or more cementing plugs stacked vertically one above the other. The cementing plug assembly is held within the housing by mechanical release devices. The release devices are separately and remotely actuated, and when a release device is activated, a plug holder is moved out of the path of a plug so that it is pulled or pushed by a combination of fluid flow, vortex action and gravity into the fluid stream where it is caught up in the moving fluid and pumped downhole. However, the Watson device projects a significant distance above the casing which necessitates much longer elevator bails than would otherwise be required. Furthermore, if it is desired to launch more than two cementing plugs, either a separate plug container that projects even further above the casing must be fabricated, or some means provided to connect the containers in series. If this is not done, the reference device provides no significant safety over earlier methods also described in U.S. Pat. No. 4,427,065) of removing and replacing a dome each time a plug is inserted, which is time consuming and expensive process that creates hazardous working conditions. Moreover, each of the various casing sizes requires use of a different housing assembly.
An object on the present invention is to provide a new and improved plug launching and monitoring system that prevents overdisplacement or underdisplacement of cement slurry during a casing cementing job. Another object of the present invention is to provide a new and improved plug monitoring system that enables reliable determination of true volumetric efficiency of the displacing means while cement displacement is in progress.